Conventionally, in an emergency state in which a far larger deceleration than that in a normal state is applied to a vehicle in a vehicle collision or the like (simply referred to as an emergency state hereafter), a seat belt device attached to a vehicle seat of an automobile or the like restrains the occupant with a seat belt. Generally, a three-point seat belt device is widely known and used as such a seat belt device. In the known general three-point seat belt device, the seat belt withdrawn from the seat belt retractor is guided by the belt guide anchor, and the tip end of the seat belt is secured to a vehicle body. A tongue is slidably supported by the seat belt guided by the belt guide anchor. In this case, the seat belt is slidably inserted through an elongated seat belt insertion hole of the tongue. When the tongue is engaged with the buckle, which is secured to the vehicle body, the seat belt is worn by the occupant.
When the seat belt is worn by the occupant, part of the seat belt between the belt guide anchor and the tongue functions as a shoulder belt worn on the shoulder and the breast of the occupant. Another part of the seat belt between an end secured to the vehicle body and the tongue functions as a lap belt worn on the lap of the occupant. In the above-described emergency state, a lock mechanism of the seat belt retractor operates so as to inhibit the withdrawal of the seat belt. Thus, the shoulder and the breast of the occupant are restrained by the shoulder belt and the lap of the occupant is restrained by the lap belt.
Generally in such a seat belt device, even when the lock mechanism of the seat belt retractor operates and the withdrawal of the seat belt is inhibited in the emergency state, the lap belt may slightly extend due to an inertial force of the occupant. Accordingly, it is unlikely that the lap belt effectively restrains the occupant.
In order to address this, the following tongue has been proposed: that is, in a normal operation state of the seat belt, friction between the seat belt and a seat belt slide surface of the tongue is decreased so as to allow the seat belt to smoothly slide against the tongue; and in the emergency state, the friction between the seat belt and the seat belt slide surface is increased so that the seat belt is unlikely to move to the lap belt side (see, for example, Patent Literature 1).
In the tongue described in Patent Literature 1, many axial-direction grooves and many circumferential-direction grooves are provided in the seat belt slide surface of the seat belt insertion hole of the tongue. The axial-direction grooves substantially linearly extend in a longitudinal direction of a seat belt insertion hole. The circumferential-direction grooves extend in a direction substantially perpendicular to the longitudinal direction of the seat belt insertion hole. In this case, the axial-direction grooves and the circumferential-direction grooves are provided entirely in the longitudinal direction of the seat belt slide surface of the elongated seat belt insertion hole. In the normal operation state of the seat belt, the seat belt slides practically against the tip ends of projections formed by the axial-direction grooves and the circumferential-direction grooves. Thus, the friction between the seat belt and the seat belt slide surface of the tongue is reduced. Thus, in the normal operation state of the seat belt, the seat belt smoothly slides against the tongue. In the emergency state, the seat belt is pulled by the inertial force of the occupant on both sides of the tongue. Thus, tension of the seat belt is increased. As a result, the seat belt is pressed into the axial-direction grooves and the circumferential-direction grooves by the increasing tension. Accordingly, the friction between the seat belt and the seat belt slide surface is increased. Thus, in the emergency state, the seat belt is unlikely to move to the lap belt side, and accordingly, extension of the lap belt is suppressed. By suppressing the extension of the lap belt in the emergency state as described above, the performance of the lap belt for restraining the occupant is improved.
Also according to Patent Literature 1, a plurality of ribs, which substantially linearly extend in the longitudinal direction of the seat belt insertion hole, is provided in the seat belt slide surface so as to project from the seat belt slide surface and to be spaced apart from one another by specified intervals in the circumferential direction. Patent Literature 1 discloses that, because of these ribs, the friction between the seat belt and the seat belt slide surface varies between the normal operation state of the seat belt and the emergency state. In this case, the friction between the seat belt and the seat belt slide surface is reduced by causing the seat belt to slide against only the surfaces of the ribs in the normal operation state of the seat belt. In the emergency state, the ribs are pressed to the lap belt side by the increasing tension of the seat belt, and the seat belt also slides against the surfaces of other parts of the seat belt slide surface. Thus, the friction between the seat belt and the seat belt slide surface is increased.
Also, Patent Literature 1 discloses that a plurality of resin members are provided on the seat belt slide surface so as to project from the seat belt slide surface. The resin members cause the friction between the seat belt and the seat belt slide surface to vary between the normal operation state of the seat belt and the emergency state. In this case, in the normal operation state of the seat belt, the seat belt slides against only the surfaces of the resin members. Thus, the friction between the seat belt and the seat belt slide surface is reduced. In the emergency state, the resin members are crushed or cut by the increasing tension of the seat belt, and accordingly, the seat belt slides against the surfaces of other parts of the seat belt slide surface. Thus, the friction between the seat belt and the seat belt slide surface is increased.